Manufacture of gasoline and coke



Nov. 1, 1938. P. OSTERGAARD' MANUFACTURE OF GASOLINE AND COKE Fil-ed NOV. 4, 1956 Patented Nov. 1, 1,938

UNITED STATES 2,135,108 MANUFACTURE oF GAsomNE AND COKE Povl Ostergaard, Mount Lebanon, Pa., assignor to Gulf Oil Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Application November 4, 1936, Serial No. 109,189

2 Claims.

My invention relates to improvements in the art of cracking heavy hydrocarbon oils for the ultimate production of gasoline and coke, and relates especially to a process of the so-called continuous coking type, wherein a heavy hydrocarbon oil unsuitable for cracking at high temperatures in a tubular or coil-type heater for the production of gasoline is heated by direct contact with hot cracked vapors, tarry materials resulting from this contact and which are derived from the initial charging stock or the cracked vapors themselves or both are discharged into an enlarged coking drum, the vapors remaining after the aforesaid contact are condensed to remove clean constituents heavier than gasoline, i. e. gas oil or cycle stock, this cycle stock is cracked in a coil-type heater at high temperatures, and the resultant highly heated cracked vapors are discharged intothe aforesaid enlarged coking chamber. In this type of operation, the hot cracked vapors effect coking of the tarry constituents delivered to the enlarged coking chamber, no liquid being Withdrawn as such from the coking chamber, and the vapors separated in the coking operation are led into contact with the fresh charging stock in the manner described. The vapors remaining after the removal of constituents heavier than gasoline are subjected to condensation for the recovery of cracked gasoline constituents. VThe foregoing is not intended as a description ofmy improved process but as a general description of the typeof process to which my invention is applied with advantageous results.

In the operation of a continuous coking unit 1 of the character described, all gas oil produced in the systemis ordinarily cracked, and all tarry constituents produced are ordinarily reduced to coke, although it will be observed that thel operation as a whole is not essentially changed by the withdrawal of a portion of the tar or a portion of the gas-oil constituents, as is sometimes done. Primarily, however, the products consist only of coke, gasoline and gases as aforesaid.

The gases contain, in addition to hydrogen, methane, ethane and ethylene, considerable quantities of hydrocarbons having 3 to 4 carbon atoms per molecule, namely, propane, propylene, butanes and butylenes; these hydrocarbons, except for a limited amount of butanes and butylenes, are too volatile to be included in a stabilized gasoline product without exceeding the permissible vapor pressure. Moreover, in units of small or moderate size, the separation'of these con-v stituents for sale as liqueiied fuel is ordinarily not commercially Worth while.

It should be observed atthis point that while operations of the character aforesaid are carried out with the deliberate purpose of making coke, a high yield of coke is not necessarily an advantage. It will be obvious to those skilled in the art that, if the production of gasoline can be increased at the expense of the coke, it will be desirable to do so, provided that the gasoline is not degraded invcharacter.V Itfmay also be observed that operations of this general type exhibit a tendency toward the production of gasoline of relatively low anti-knock value, due possibly to the fact that a considerable portion of the gasolineA is lproduced as a result of the liquidphase cracking which takes place as an incident to the coking; as a general rule gasoline produced in liquid-phase cracking operations tends to be of lower anti-knock value than gasoline produced in vapor-phase cracking operations.

In my prior co-pending applications Serial No. 52,717, Iiled December 3, 1935, and Serial No. 97,295, iled August 21, 1936, I have disclosed various processes of cracking hydrocarbon oils,

wherein hydrocarbons having 3 to 4 carbon atoms per molecule produced in the system as well as,

where desired, similar hydrocarbons from an exV traneous source, are combined with the oil chargedto the conversion units. 'I'his form of gas recycling, when properly carried out, as dscribed in detail in my application Serial No. 97,295, is highly advantageous in that it provides for the production of an increased yield of gasoline over that which would ordinarily be obtained, and the gasoline is of high anti-knock value. The best results are-obtained, however, insofar as light vstocks are concerned, only when the cracking of the oil in the presence of re-l cycled gases is carried to a degree of conversion per pass'higher than could be obtained ifthe oil were subjected to conversion alone in the same apparatus in the absence of the gases.

In this type of gas-recycling operation, the oilis cracked to a higher extent than would otherwise be possible, the gases themselves, particularly saturated constituents thereo'f, exhibit some tendency to crack, unsaturated constituents of the gases tend to polymerize to gasoline-like hydrocarbons, and reactions take place involving saturated or unsaturated constituents Iof the gases and products 'of conversion of the oil unn dergoing cracking; the iinal result being that the yield of tar is reduced and there is obtained there are ordinarily two a high yield of gasoline of superior anti-knock value.

I have now found that extremely desirable results may be obtained by effecting reaction at high temperatures and pressures between tarry constituents and hydrocarbons containing 3 to 4 carbon atoms per molecule; this reaction may also involve to some extent reaction of the tar with gaseous constituents of even lower molecular weight, although methane is too refractory in character to enter into reaction to any substantial extent under practical operating conditions. The reactions are particularly effective if the normally gaseous hydrocarbons are brought into contact with the tar during the course of molecular change, that is to say during the course of formation of the tar, or during subsequent cracking of tarry constituents.

not ordinarily feasible to subject tar to cracking in the presence of recycled or added gases, while traversing a tubular heating coil; the diluent effect of the gases tends slightly to increase the temperature to which the tar can be subjected in the coil without carbon deposition, but this effect, insofar as tar is concerned, is not strong enough to make it possible to obtain optimum results, as distinguished from those results which can be obtained by heating relatively clean stocks in the presence of added normallygaseous constituents.

For these reasons and for certain other reasons which will be set forth hereinbelow, I have found that extremely advantageous results may be secured by incorporating gas recycling as set forth in my co-pending applications referred to hereinabove, in a unit of the above-mentioned continuous coking type. My invention in one specie aspect, 'therefore contemplates the recovery of C3 and C4 hydrocarbons from the resulting cracked gases by scrubbing them with'a portion or all of the gas oil produced in the system, this gas oil being cooled for absorption purposes; the enriched gas oil is then delivered to the hightemperature cracking coil and the products of conversion of the oil and gases are then introduced intothe enlarged coking drum where they .are contacted with tarry constituents supplied to the coking drum for coking.

` It may also be observed at this point that in operating a continuous cracking process running to coke, gasoline and gas as ultimate products, factors which definitely limit the yield of gasoline within a motor-fuel boiling-point range, at least when carried out without benefit of my present invention. In the rst place', the coking process necessarily produces a considerable amount of normally gaseous hydrocarbons too light to be incorporated in the final stabilized motorfuel or gasoline, a result influenced to a, considerable extent by the h igh cracking temperatures to which the recycle gas oil is subjected. In the second place, the cycle stock produced in a continuous coking operation tends to be very refractory in character, and when cracked'tends to produce a large amount of tar and a large amount of gas, with a relatively poor yield of hydrocarbons within the motor-fuel boiling-point range. This refractory character of the recycle stock is due at least in part to the fact that it is primarily composed of naphthenic and aromatic compounds with a low hydrogen to carbon ratio.

In accordance with my present invention, the disadvantages of a continuous coking operation of the type described are largelyovercome. In

However, due to the high' carbon residue values of tarry constituents, it is lof such exempliflcation.

the irstplace, a considerable portion of the C: and C4 hydrocarbons in the resultant gas are converted directly bypolymerization or by cracking and polymerization to normally liquid hydrocarbons lying within the motor-fuel boiling point range and, having exceptionally high anti-knock value andeven.higher-anti-knock blending value. In the 4second place;- the conversion of the refractory recycle stock is improved, d ue to the presence of the recycledgases, with the result ofthe productionA of, .hgher yield of gasoline-like hydrocarbons from the recycle stock; the recycle stock itself tends to beA of somewhat l-ess refractory character, due to reaction of, naphthenic and aromatic compounds with the gases present. Third, not only is the initial production of tar reduced, due to the reactions mentioned above, but when such tar as is produced is returned to the coking operation and there subjectedr to coking and cracking temperatures in the presence'of gases, combinations between constituents of the tar or constitutents resulting from cracking ofthe tar with normally gaseous constitutents take place, resulting in the production of intermediate compounds. The latter include gasoline of high value as well as gas-'oil constituents of excellent character for subsequent cracking to gasoline. The

entire operation is improved. Moreover, it is not invention has for objects such additional operative improvements and advantages as may hereinafter be found to obtain.

Inorder that my invention may be more clearly set forth and understood, I now describe, with reference to the drawing accompanying and forming a part of this specification, various preferred forms and manners in which my invention may be practiced and embodied, but without limiting my invention in its broader aspects to the details In this drawing,

Figure l is an elevational view, in more or less diagrammatic form, of apparatus for cracking hydrocarbon oil in accordance with the process of my invention; and

Figure 2 is a similar view of an alternative form of a'portion of the apparatus illustrated in Figure 1.

Similar reference numerals designate similar parts in both of the views of the drawing.

Referring now to the drawing, and more particularly Figure 1 thereof, the major units of the apparatus shown comprise a cracking coil I located in a suitable furnace setting, an enlarged coking drum 2, a separating tower 3 having an upper section 4 land a lower section 5, a fractionator 6, a vapor-feed condenser and stabilizer 1 `having a condensing section 8, a stabilizing sec-V tion 9 and a reboiler I0, and an absorber Il, together with incidental lines, pumps, valves, heat exchangers, accumulators and the like as Will be 'described more fully'hereinbelow. Vapors liberated in the cracking operation conducted in the -residual petroleum oil or a pressure-still tar, is

900 F. at the top of the. same.

acter, a plurality of coking drums 2 may be provided, arranged to series or parallel or alternate now, so that while coke is accumulating inone of the drums the other drums may be disconnected from the system for coke removal or for preheating. However, in the interests of simpliiication, a single coking drum 2 has been shown inthe drawing.

:I'he oil to be cracked, which may consist of a gas oil, a heavy crude, a reduced crude, a heavy introduced into the system through a line I2 and is delivered by a pump I3 through a line I4, a heat exchanger I and a line I6 to the upper section 4 of the separating tower 3, where this oil, which has been preheated to some extent by indirect contact with hot vapors in the heat exchanger I5is further heated and distilled by means of hot cracked vapors risingthrough `the separating tower 3 from a source to be described hereinbelow,V Theupper section 4 of the separating tower-3 is .interiorly provided with suitable plates or trays I1 and is separated from the lower section 5 of the separating tower 3 by means of a trap tray or accumulator section I8 of conventional design. In the section 4, thelighter components cf the charge oil are distilled overhead, passing out of the separating tower 3 through a line i8 to i the heat exchanger I5 and thence through a line 20 into the fractionator 6, .while the residual constituents pass downward through the section 4 to thejaccumulator section I8, together with heavy 'ends condensed from the cracked vapors. The oil thus accumulating in the section,|8 is removed therefrom by means of a line 2| and, where it is desired to coke only this oil and not the heavy tar produced in the cracking and coking operations, this oil is then delivered through a line 22 having' a valve 23 and a line 24 to a pump 25 which delivers the oil through a. line 26 into a transfer line 21 leading from the cracking coil I into the coking drum 2,

or directly to the coking drum 2,

In the coking drum 2, which is maintained under a pressure of from 100 to 1000 pounds per square inch, the heavier constituents in the hot charge from the section I8jof the separating tower 3 and the heavy constituents in the cracked vapors from the cracking coil I are reduced to coke with the production of relatively lower boiling hydrocarbons includingvgases,A pressure-still distillate or gasoline and recycle gas oil, as well as some tar. The temperatures will depend upon the pressure to some extent, but at pressures of from 100 to 1000 lbs. per square inch, the temperatures will run from '150 to 950" F. at the bottom of the coking drum 2, and from 700 to c The coke is deposited in the coking chamber2, while the oils boiling below 900 F. are passed overhead in vapor form from the coking drum 2 through a vapor line 28 having a valve 29 into the lower section 5 of the separating tower 3. At the outlet of the coking drum 2, cooled low boiling oil, such as gas oil from another unit of the apparatus, is preferably introduced into the vapor line 28 through aline 30 for the purpose of controlling the temperatures of tire vapors vin the line 28 and preventing coking in the vapor line 28.

The vapors enter the lower section 5 ofthe separating tower 3 through a line 28 and a pressure-reducing valve 28 and are there separated into tar and vapors, this operation being assisted,

In the'section 5, which is v preferably provided with plates or traysy 32 and bailles 33, heavy hydrocarbon oils or tars boiling between, say, 800 F. and 900 F. are deposited in the base of the tower as tar, while the lighter vapors pass upward into the section 4. Where it is not desired to reduce the tar to coke,- the tar may be withdrawn from the bottom of the section 5 through a line 34 leading to a tar cooler 35` and thence discharged from the system through a line 36. is desired to reduce the tar thus formed to coke, and thus avoid the production of any tar as an ultimate product, the tar is withdrawn from the bottom of the section 5 through a line 31 'and delivered by means of line 31 and the line 24 to the pump 25 which delivers it into the coking drum 2. In this instance the trap tray I8 may be omitted or the heavy oil accumulating in the section I8 may be delivered to the lower section 5 of the separator 3 through the line 2| and a valved lline `38. Where this is done, the oil re- -moved at 31 ani returned to the coking drum 2 On the other hand, when it contains all of the tarry constituents recovered from the vapors as Well as the residual constituents ci the distilled charging oil. c

The vapors introduced into the upper section 4 of the separating tower 3 are' subjected to partial condensation by contact with the charge oil introduced thereto and also, if desired, by means of cool reilux oil introduced into the top of the section 4 through a line 39, heavy recycle oil being condensed and returned to the coking drum 2 along with the residual components of the charging oil, as described hereinabove.

In this manner all heavy residual oil in the charge can be completely converted to coke, light recycle oil, pressure-still distillate and gas, while -tar produced in the cracking and coking operation is recycled in the coking Zone and there reduced to coke or reacted to form lower boiling constituents, or it is removed from the system if desired. While it is the general object of the present invention to operate to ultimate yields of coke and gasoline, it is recognized that the quality of the' coke produced will be influenced to some extent by the introduction of cracked tar into theI coking drum 2; where a hard, dense coke 4is desired or where insumcient heat is available light recycle cil, `pressure-still distillate or gasoline, xed gases and low-boiling hydrocarbons lintermediate between gasoline and the fixed gases. These vapors pass through the heat exchanger I5 in which they are partially cooled by heat exchange with the incomingcharge, thereby preheating the latter. The vapors then 'pass through the line 20 into the fractionator 6, the purpose of which is to condense and remove recycle stock heavier than gasoline but substantiallyI free of tarry constituents and hence representing a cycle stock which may be subjected to rconversion in the coil I for the production of gasoline at high temperatures and with a high degree of conversion per pass.

The fractionator 6 isv interiorly provided with suitable plates or trays'40 and with suitable cooling and refluxing means. In the instance illustrated in the drawing, a side stream is withdrawn from a point near the top of the fractionator 6 through a line 4I. After passing through a cooler or heat exchanger 42 the cooled side stream is delivered by means of a pump 43 and a reflux line 44 to the upper portion of the fractionator 6, thereby assisting in the fractional condensation operation taking place in the fractionator 6. That operation is so conducted as to reduce the temperature of the vapors to such a point that all constituents heavier than are desired to be included in the nal gasoline product are condensed as gas oil or cycle stock. The condensate thereby obtained accumulates in the bottom of the fractionator 6 and is Withdrawn therefrom through a line 45 leading to a pump 46.

The vapors leaving the fractionator 6 pass through a vapor line 41 to the` condensing section 8 of the vapor-feed condenser and stabilizer 1, the purpose of which is to condense and recover stabilized gasoline distillate of the desired volatility and boiling pointrange. While more conventional forms of condensing and stabilizing apparatus may be used with less advantageous results, I have found that the specific form illustrated is of especial advantage in my process, where the gases and vapors treated contain an exceptionally high proportion of constituents lower boiling than gasoline. This specific type of vapor-feed condenser and stabilizer forms the subject matter of my co-pending application Serial No. 103,947, filed October 3, 1936, and is also referred to and described in conjunction with gas-recycling oil-cracking operations in each of my other co-pending applications referred to hereinabove.

In the condensing section 8, condensation is effected by withdrawing a stream of unstabilized gasoline from a trap tray 48 located between the condensing section 8 and thestabilizing section 9, through a line 49. The condensate thereby withdrawn passes through a cooler 5D and a line 5I to a pump 52, which in turn delivers the cooled unstabilized condensate through a reux line 53 into the top of the stabilizing section 8 'i as a cooling and reiiuxing medium.

A further portion of the unstabilized gasoline condensate passes downward from the trap tray 48 through an overflow line 54 into the stabilizer section 8 which is connected by means of liquid and vapor lines 55 and 5,6, respectively, to the reboiler I0. Sufficient heat is supplied to the reboiler I Il to effect partial redistillation and stabilization of the gasoline. Vaporsliberated during the course of the stabilization pass up- Ward rinto the condensing section 8. The stabilized gasoline passes out of the reboiler I through a line 51 to a cooler 58 and is then removed from the system through a line 58. In the instance shown, the heat required in the reboiler Il) is advantageously supplied by delivering a part or all of the stream o f hot gas oil condensate from the fractionator 6 through the line`45, the pump 46 and a line 60to a heating coillocated within the reboiler I0, but theneces- .sary heat may be otherwise supplied. l

The gases and vapors leaving the top of the condensing section 8 through a line 6| consist of methane, ethane, ethylene, propane, propylene, butanes and butylenes in varying proportions and may also contain some hydrogen, as well as; very small amounts of hydrocarbons containing or more carbon atoms per molecule.

These gases, together, if desired, with additional hydrocarbon gases introduced through a line 82 and containing hydrocarbons having 3 to 4 carbon atoms per molecule, then pass through 'a cooler 63vand a line 64 into the lower part of the absorber II. Condensate produced in the cooler 63 may also be delivered through a trapped linefy the higher-boiling constituents thereof; primarily butanes and butylenes as well as a portion or all of the propane and propylene and, if desired, some ethylene are absorbed. The scrubbed gases leave the upper part of the absorber II through a gas line 12 having a back-pressure valve 13, while the enriched gas oil passes from the bottom of the absorber II through a line 14 into an accumulator 15, which is provided with a vent line 16 leading back into the absorber II.

From the accumulator 15, the enriched gas oil or recycle stock containing absorbed normally gaseous hydrocarbons passes through a line 11' to a pump 18, which delivers it through the heat exchanger 61 and a line 19 into the cracking coil I. In the coil I, the enriched oil is subjected to cracking under high-temperature conditions, the conditions of pressure, temperature and cracking time being preferably so adjusted as to effect a higher degree of conversion per pass than could be obtained if the oil alone were subjected to cracking in the absence of the absorbed normally gaseous hydrocarbons, in the same apparatus; the cracking is' carried out largely in the vapor phase.

In general, the temperatures, other conditions being the same, which are maintained in the heating coil I will be from 50 to 200 F. higher than would be feasible for the cracking of the oil in the absence of the gases, and while the specific temperatures will vary somewhat depending upon the results desired, the gas recycle ratio, the type of apparatus, the contact time and other factors, I have found that coil-outlet temperatures varying from 950 to 1250 F. are suitable, with pressures of from 100 to 2000 pounds per square inch at the outlet of the coll. Pressure is maintained by means of a valve 88 located in the transfer line 21.

That portion of the cooled gas oil leaving the cooler 68 which is not employed as an absorbing agent in the absorber II is employed partly as a quenching medium and partly as a refluxing medium in various portions of the apparatus: For use as a quenching medium and as a means of regulating the temperature in the coking drum 2, a portion of the cooled gas oil is delivered through a line 8|, a line 82, a pump 83 and a quench line 84 to the transfer line 21 at a point close to the outlet from the heating coil I, the amount so introduced being suflicient to regulate the temperature in the coking drum 2, and to prevent coking in the transfer line 21. Another portion of the cooled gas oil may be vdiverted from the line 8| through the line 38 into the vapor line 28 as described hereinabove, -while still further portions may be diverted from the line BI through a line 85, thence passing through 1| medium in the absorber is removed through a line BI passing to the pump 4G andthence to the cooler 68 and absorber II. Cooled recycle oil is introduced as a reflux to the fractionator 6 through a reflux line 92 communicating with the line 8|, and thereby providing for the-condensationv in the lower part of the fractionator 6 of a heavy recycle oil, which leaves the fractionator 6 through a line 93. In this instance,

the heavy recycle stock removed at 93 is withdrawn from the system and separately cracked,

or it is used elsewhere in the system as a quenching medium or for heat-exchange purposes.` It: will be observed that this modification provides for additional iiexibility of operation and for the delivery to the absorber and the cracking coil of a. somewhat lighter fraction than would be true in operation in accordance with Figure 1 and the attendant description.

In operating my process, the extent to which absorption is carried in the absorber will always be such as to eifect the removal of all butanes or butylenes from the gases traversing the absorber, 'and insofar las the gases produced in the process are concerned, it will ordinarily be\desirable to eiect also the absorption and system through the line 62, this proportion of.

propane and propylene in the dry gas may not hold true. The absorption, in any event, should be so conducted as to deliver to the cracking coil I a mixture of gases and normally gaseous hydrocarbons in a ratio of from 1:4 to 4:1, such ratios Y being the ratios of liquid volume of liqueiied gas to liquid volume of the oil, measured under atmospheric conditions.

As will be clear from the description set forth hereinabove, the saturated constituents Vin -the gases introduced into the cracking coil tend to undergo cracking, while the unsaturated con- AIsls stituents introduced as such ork resulting from the cracking of the saturated constituents tend to' cracking of the oil and to react withproduc'ts` of cracking in such manner as to reduce the ultimate formation of tar constituents and toproduce oils of intermediate boiling-point range, such as gasoline and recycle stock of less refractory character than would otherwise be produced.

In the coking drum 2, further reactions take place, primarily involving a reaction of gas constituents unreacted in the cracking coil I or;`

produced in the cracking coil I, or introduced directly into the drum 2, with residual or tarry oil constituentsintroduced into the coking vessel 2,

as well as with constituents formed during ther sidual or tarry constituents react with gases to acter as eventually to cause a reduction in the amount of tarryconstituents, gas and coke produced, an increase in theamount and quality of the gasoline produced, and a reduction in refractoriness of the cycle stock constituents pro- 5 duced in the operation, with the overall result that the process produces a higher yield of a higher anti-knock value gasolinaand lower yields of fixed gas and coke than would be produced without benet of my invention. All of these results are obviously advantageous.

As a modication of my invention, that portion of the normally gaseous hydrocarbons which is condensed in liquid form in the cooler or condenser 63, or a portion of the condensate thereby obtained, may be delivered directly vto the coking drum 2 instead of being passed through the absorber I and subsequently introduced along with the gas oil into the cracking coil I. For this purpose I provide a valved line |20, having a pump I2I. ,l This operation is advantageous in that the butanes and butylenes of which such condensate is primarily composed, as well as such small amounts of higher boiling hydrocarbons as may be contained in said condensate, are especially suitable for reaction with the naphthenic and aromatic constituents of the tar having a low hydrogen carbon ratio. o

With further reference` to the specific process described in connection with Figures 1 and 2, it' 30. will ordinarily be desirable to operate the towers 3, 6,-1 and I I'under pressures varying from 100 to 500 pounds per square inch, with only'such reduction of pressure as is incident to the normal back pressures of these towers. The "coking op'- eration is, from a practical standpoint, best car ,Y

ried out to give a coke containing from 5 ,to l5 -per cent of volatile matter; in order to keep-the coke within these limits, it may in some instances be necessary to divert from the system a portion of the tar produced, as, for example, by removing tar from the lower section 5 of the separating tower 4 through'the line 34; Ordinarily, however, due to the fact that the presence of the gases inthe cracking coil I makes it not only possible but desirable'to carry out the cracking operation at a higher temperature than would be practical in the absence of the gases, and due to' the fact that a considerable portion of the reform lower boiling oils rather `than coke, sufficient heat is available to accomplish the coklng of all of the tar produced in the operation, down to a coke having a desired `content of volatile matter. l f p.

In some instances the charging stock may be delivered directly to the coke drum 2 instead of to the separating tower 4. For this purpose, I provide a valve |00 located inthe line I6, and a line IIII, having a valve |02, and leading frornthe line I6 to the line 24. In this'instance, the charging stock leaving the heat exchanger I5 passes through line I6, line IIII, pump and line 26 into the transfer line 21, and therein into the` coke drum 2.

It will be obvious to those skilled in the art that Y ,while \I have described my invention hereinabove with respect to a specic apparatus assembly and lwith .reference to various illustrative details of l ,operation, my invention is not in its broadest aspects limited to such details 'but may be variously practiced and embodied within the scope of the cracking of these residual or tarry constituents. claims hereinafter made. Thus the specific con- It will be obvious that the reactions are extremely complex and involved but they are of such charstructional details of the apparatus shown and the specific heat-exchange arrangements illustrated may be varied to suit individual conditions, although those specifically illustrated are preferred and advantageous. Various modiiications which do not affect the underlying principles of my invention will undoubtedly suggest themselves to those skilled in the art but are intended to be contemplated within the scope of the claims hereinafter made.

What I claim is:

1 The process of cracking heavy hydrocarbon oil containing Vresidual constituents in such amount as to develop high carbon-forming tendencies when subjected to cracking temperatures, for the recovery of a high yield of cracked gasoline oi" high anti-knock value, and coke, which comprises: introducing said oil into a distilling zone and there contacting it with highly heated vapors and gases, thereby vaporizing non-residual constituents of said oil, and collecting residual constituents of said vapors in the distillation residue; fractionating the resultant mixed vapors and gases in successive steps to recover a clean gas-oil condensate substantially free from gasoline constituents and a cracked gasoline distillate; scrubbing uncondensed gasoline-'free vapors and gases with at least a portion of saidgas-oil condensate to recover normally gaseous hydrocarbons having 3 to 4 carbon atoms per molecule by absorption in said oil, discharging the unabsorbed arcaica gases from the system; subjecting the enriched gas-oil condensate from the scrubbing zone to substantial conversion in an externally iired elongated cracking zone of restricted cross-sectional area at a high cracking temperature substantially in excess of the maximum temperature to which the oil alone'and Without admixture of said normally gaseous hydrocarbons could be subjected in identical apparatus and under otherwise identical conditions of conversion Without such excessive deposition of carbon as to prevent continuous operation of the unit for extended periods of time; passing the resultant hot cracked vapors together with distillation kresidue oil from said distilling zone into an enlarged coking chamber, and there reducing said residue oil to coke; Withdrawing hot gases and vapors from said coking chamber, and introducing them into said distilling zone as aforesaid.

2. A process as set forth in claim 1, in which the gases and vapors, after condensation of gasoline and prior to scrubbing with the gas-oil condensate, are cooled under pressure to recover a .fraction consisting of iiqueed higher boiling con- 

